1. Field of the Invention
The present invention relates to a method and apparatus for evaluating damage of metal materials, and particularly relates to an evaluation method of metal materials suitable for evaluating a brittle creep damage generated at the welded portion of low-alloy steels used for high temperature-high pressure resistant metal members such as boilers in thermal power plants.
2. Background Art
Operating hours of power plants have recently been increasing. Such long operating hours, frequent starting and stopping operations, and rapid load fluctuations of power plants cause creep fatigue of power plant facilities and degrade the plant facilities. In order to cope with degradation due to the thermal fatigue of facilities in power plants, an emphasis is placed on a maintenance technology of power plants, considering creep fatigue of metal components.
For example, thick and large diameter pipes made of high temperature-high pressure resistant steel metal materials are subjected to damage such as cracks which originate, in most cases, in the internally in welded portions. However, since the damage such a crack is difficult to detect externally, development of technology for early detection of the damage such as a crack and a monitoring technique of the crack by an accurate measurement of the crack are problems to be solved.
Conventionally, the crack depth from the surface is measured by an edge echo method based on an ultrasonic flaw detection analysis.
However, in order to detect the crack depth using the edge echo method, it is necessary for an operator to determine the crack depth by reading the subtle change of the edge echo, so that the problem arises that the result of the edge echo method is liable to include personal errors. A method called TOFD (Time of Flight Diffraction technique) is used for detecting the internal defects such as cracks and for determining the quantitative values such as the length of the defect.
FIG. 11 is a diagram for explaining the principle of the measurement of the TOFD method. The TOFD system comprises a transmitting probe 1 for transmitting a ultrasonic wave and a receiving probe 2 for receiving the ultrasonic wave. During measurement by the TOFD system, the transmitting probe 1 and the receiving probe 2a are mounted on a metal sample 3 which includes a crack (defect) 4 such that the crack is positioned in the middle of the transmitting and receiving probes and an ultrasonic wave is transmitted obliquely by the transmitting probe 1 towards the crack 4 in the metal plate and the diffracted waves 6 from both of the upper and lower edges of the crack are detected by the receiving probe 2 for measuring the propagation time of the ultrasonic wave. The height of the crack 4 is obtained by measuring the difference between the propagation from the top of the crack and that from the bottom end of the crack by the following equation (1).                     L        =                  Zb          -          Zt                                        =                                            (                                                                    tb                    2                                    ·                                      V2                    /                    4                                                  -                S2                            )                                      1              /              2                                -                                    (                                                tt2                  ·                                      V2                    /                    4                                                  -                S2                            )                                      1              /              2                                          
where, L represents the height of the crack; Zb represents the depth of the top of the crack; Zt represents the depth of the crack bottom; D represents the distance between the transmitting and receiving probes; S represents D/2, V represents the velocity of the diffraction wave; tt represents the propagation time of the diffraction wave from the top end of the crack; and tb represents the propagation time of the diffraction wave from the bottom end of the crack.
The above-described TOFD method has the advantage that, since this method measures the diffracted waves from the crack, it is possible to reduce the effect due to slanting of the crack or to reduce the possibility of overlooking directional defects and, as a result, the performance in detecting the defects is improved.
However, when an alloy steel which has been used for 10 to 20 years is examined by the TOFD method, the problem arises that, since it is not possible to determine whether the detected defect is caused as a creep damage due to the aged deterioration or caused in the manufacturing process, the estimation of the remaining service life is difficult.
For example, when a low alloy steel, usually used for thick pipes having large diameters, is examined, it is found that the creep crack growth rate is dependent on the material composition of the low alloy steel, and the impurity content of the metals used for the welded portion. In particular, it is known that the crack growth rate is dependent on the thermal stress for circumferentially welded portion. Furthermore, it was found that the rack develops not at the outside of the pipe but inside of the steel pipe where high stress is applied multiaxiality.
If it is possible to determine whether the crack inside of the pipe is developed by creep damage or generated in the manufacturing process, it becomes possible to estimate the remaining service life of the steel pipe. No study has been reported until now.
The present invention was made to solve the above problems. It is therefore an object of the present invention to provide a damage evaluation method and the apparatus thereof, which is capable of determining whether an internal crack of a metal material is developed by the creep damage by an aged deterioration or caused in the manufacturing process, and which is capable of evaluating the remaining service life of a metal component.
According to the first aspect of the present invention, a damage evaluation method of a metal material for evaluating a flaw in the metal sample comprises the steps of: mounting onto a meal surface including an internal flaw and on both sides of the internal flaw a transmitting probe for transmitting an ultrasonic wave and a receiving probe for receiving the ultrasonic wave; transmitting the ultrasonic wave towards the internal flaw and receiving the diffracted wave from the internal flaw for determining whether or not a flaw is present in the metal.
In this method, when a flaw is present in a metal sample, an ultrasonic wave transmitted toward the flaw by the transmitting probe is diffracted by the flaw as propagate in the metal sample giving rise to diffracted wave. If the diffracted wave is detected, it is determined that a flaw is present in the metal sample.
According to the second aspect, in the damage evaluation method of a metal material for evaluating a flaw in the metal sample according to the first aspect, it is determined whether the flaw is caused by creep, through metallographic analysis after carrying out the metallographic analysis of the metal surface in which the internal flaw is present.
This method carries out a metallographic analysis for the surface of the metal sample by obtaining a replica of the metal sample and makes it possible to accurately estimate whether the flaw is due to creep by evaluating whether the flaw is present and whether the metal microstructure is degraded.
According to the third aspect, in a damage evaluation method of a metal material for evaluating a flaw in the metal sample, the method comprises the steps of: conducting chemical analysis of a sample taken from the surface of said metal; estimating the creep characteristics of the metal based on the results of the chemical composition analysis; carrying out stress analysis based on the creep characteristics; and determining whether or not said flaw is due to the creep based on the stress analysis.
In this method, a small piece of sample is taken from the surface of said metal sample, the impurity concentrations are determined by the chemical analysis, the creep characteristics of the metal material are estimated, and the stress analysis is carried out based on the creep characteristics. In this stress analysis, since a damage estimate representing the metal sample ductility is obtained, it becomes possible to determine whether the flaw is caused by creep based on the above obtained damage estimate.
According to the fourth aspect, a damage evaluation method of a metal material for evaluating a flaw in the metal sample comprises the steps of: performing a chemical analysis for a sample taken from a surface of said metal; extracting crack propagation data suitable for said metal material from the result of said chemical analysis and a predetermined relationship between impurity concentrations and corresponding crack propagation rate; and determining the remaining service life of the metal sample from the crack propagation rate.
In this method, the impurity concentrations are obtained by the chemical analysis of a small piece of sample obtained from the surface of the metal sample, and crack propagation data suitable for the metal sample is obtained by the impurity concentration and predetermined relationships between the impurity concentrations and the crack propagation rate by the creep. Since the crack propagation data includes the distance of the flaw to the surface of the metal sample, and the crack propagation rate, the time period for the crack reaching the surface of the metal sample can be obtained from the crack propagation behavior shown in the graph representing the relation between the crack height and time. Thus, the remaining service life of the metal sample can be estimated from the crack propagation data.
According to the fifth aspect, a damage evaluation method of a metal material for evaluating a flaw in the metal sample comprises the steps of: carrying out a metallographical analysis for the surface of the metal; estimating the degree to which creep damage has progressed; and extracting the crack propagation data from the thus-estimated degree to which the creep damage has progressed and from the predetermined relationship between the degree to which the creep damage has progressed and the creep crack propagation rate; and determining the remaining life of the metal sample containing the flaw.
In this method, the surface microstructure of the metal sample is analyzed by the replication method, the degree to which the creep damage (creep damage degree) has progressed is estimated based on the metallographical analysis, and the crack propagation data suitable for the metal sample is extracted based on the above estimated creep damage degree, and the predetermined relationship between the creep damage degree and the crack propagation rate. Since the crack propagation data include distance of the flaw to the surface of the metal sample, and the crack propagation rate, the time period for the crack reaching the surface of the metal sample can be obtained as a time when the crack height is equal to the wall thickness. Thus, the remaining service life of the metal sample can be estimated from the crack propagation data.
According to the sixth aspect, a damage evaluation method of a metal material for evaluating a flaw in the metal sample is provided, wherein the damage evaluation method for evaluating a flaw in the metal sample uses the damage evaluation method according to the first aspect, the damage evaluation method according to the second aspect, and the damage evaluation method according to the fifth aspect.
This method makes it possible to determine whether the flaw is caused by the creep damage and to estimate the remaining service life of the metal sample with the flaw.
According to the seventh aspect, a damage evaluation method of a metal material for evaluating a flaw in the metal sample is provided, wherein the damage evaluation method for evaluating a flaw in said metal sample uses the damage evaluation method according to the first aspect, and the damage evaluation method according to the third aspect.
This method makes it possible to determine accurately whether the flaw is caused by the creep.
According to the eighth aspect, a damage evaluation method of a metal material for evaluating a flaw in the metal sample is provided, wherein the damage evaluation method for evaluating a flaw in said metal sample uses the damage evaluation method according to the first aspect, and the damage evaluation method according to the third aspect, and the damage evaluation method according to the fourth aspect.
This method makes it possible to determine whether the flaw is caused by the creep damage and to estimate the remaining service life of the flaw.
According to the ninth aspect, a damage evaluation apparatus of a metal material for evaluating a flaw in the metal sample is provided, wherein the damage evaluation apparatus for evaluating a flaw in the metal material comprises: a creep characteristic estimation device for estimating the creep characteristics of said metal material based on the chemical analysis of the metal sample taken from the surface of said metal sample; a determining device for determining whether or not said flaw is originated by the creep by carrying out the stress analysis based on said creep characteristics of the metal material.
This apparatus makes it possible to determine in a more accurate and prompt manner whether the flaw is caused by creep.
According to the tenth aspect, the damage evaluation apparatus of a metal material for evaluating a flaw in the metal sample comprises: a data extracting device for estimating the crack propagation data suitable for said metal material from the results of the chemical analysis of the surface of said metal sample and from the relationship between predetermined impurity concentration and crack propagation velocities; and a remaining service life estimating device for estimating the remaining service life of the metal material including the flaw.
This apparatus makes it possible to determine in more accurate and prompt manner whether the flaw is caused by the creep.
According to the eleventh aspect, the damage evaluation apparatus of a metal material for evaluating a flaw in the metal sample comprises: a creep damage estimation device for estimating the degree to which the creep damage has progressed based on the results of the metallographical analysis of the surface of said metal sample; a crack propagation data extracting means for extracting the crack propagation data suitable for said metal material from the thus estimated degree to which the creep damage has progressed and from the relationship between a predetermined degree to which the creep damage has progressed and the crack propagation rate; and a remaining service life estimation device for estimating the remaining service life of said metal material.
This apparatus makes it possible to estimate in more accurate and prompt manner the remaining service life of the metal sample.